The introduction of copper (Cu) metal into multilayer metallization schemes for manufacturing integrated circuits, enabled by the Damascene copper plating process, is now extensively used by manufacturers of advanced microprocessors and application-specific chips. As the minimum feature dimensions on patterned substrates (wafers) steadily decreases, several consequences of this downward scaling are becoming apparent. One concern is the extendibility of physical vapor deposition (PVD) to produce copper seed (nucleation) layers for electrochemical plating of the substrate and the features formed on the substrate. These seed layers must become increasingly thinner due to the decreasing minimum feature sizes, and unless significant improvements in the PVD-Cu step coverage are made, it may not be possible to achieve robust continuous film of copper onto these features (e.g., on the lower sidewall of a via hole) to permit consistent nucleation of electrodeposition.
Alternatives to PVD-Cu seed layers for copper electroplating include non-Cu seed layers, for example ruthenium (Ru) seed layers that can be highly conformally deposited by chemical vapor deposition (CVD) or atomic-layer deposition (ALD). One challenge that Cu and non-Cu seeds pose to electrochemical plating processing and the plating equipment is non-uniform thickness of the plated Cu metal onto the seed layer due to the terminal (‘resistive substrate’) effect. The terminal effect is the tendency for the current density to be non-uniform as a result of the ohmic potential drop associated with conducting current from the wafer edge to the entire wafer surface through a thin resistive seed layer. This problem can be more severe for a highly resistive non-Cu (e.g., Ru) seed layer than a lower resistivity Cu seed layer. The sheet resistance of a non-Cu seed layer can be orders of magnitude higher than that of today's Cu seed layers and straightforward extension of methods currently used to manipulate current distribution (e.g., electrolyte conductivity) generally will not be adequate to combat the terminal effect experienced using a non-Cu seed layer. Therefore, new methods are required to improve the uniformity of a copper metal layer electrochemically plated onto a non-Cu seed layer.